The protozoan Tetrahymena will be used as a model organism to study the effect of incorporation of a sterol different from the native compound on enzyme activities in the organism. Specifically, the normal membrane component tetrahymanol will be replaced by ergosterol (the native sterol of yeast). The normal and the sterol-replaced cells will be compared with respect to the activity and characteristics of the enzyme system which produces olinoleic acid in this organism. This enzyme system appears to require the coenzyme A derivative of the substrate, oleic acid, and is therefore referred to as the oleoly CoA desaturase. Various kinetic properties and the susceptibility of the enzyme system to changes in the endoplasmic reticulum lipid composition will be studied. In addition, the enzyme will be purified, and the quantity of enzyme present in both cell types (specific activity) will be determined to find out whether the evident decrease in activity in ergosterol-grown whole cells is due to changes in activity of the enzyme or to amounts of the enzyme present within the membrane.